The Impact of Glaciation

The direct influence of continental glaciers upon the surface of the South Central US is limited because even during the most extreme intervals of Pleistocene glaciation, the Laurentide Ice Sheet barely reached as far south as northern Missouri and Kansas (see Figure 6.1). Glacial deposits that exist in Missouri and Kansas can be difficult to date to a specific glacial advance and are simply labeled as pre-Illinoian. There were perhaps five pre-Illinoian glaciations in Missouri, representing glaciations as recently as a few hundred thousand years ago, and two in Kansas, the most recent being about 600,000 years ago.

Figure 6.12: Ice core atmospheric temperature and carbon dioxide concentrations from an ice core taken in Vostok in Antarctica along with with CO₂ data from several cores in Greenland give a record of glacial advances over the past 800,000 years. Glacial deposits in the South Central are from pre-Illinoian advances.

Glacial drift, including large numbers of rocks from outcrops that occur farther north, is the primary evidence that remains of these glaciers (Figure 6.13). Larger glacial landforms that probably existed have since eroded away. Among the most distinctive glacial erratics found in the South Central are quartzite boulders, known as the Sioux quartzite, dragged by glaciers from the area of Sioux Falls, South Dakota (Figure 6.14). Perhaps because quartzite is so resistant to erosion, boulder fields of this rock can be especially prominent.

Figure 6.13: The distribution of glacial debris in Kansas and Missouri, marking the southernmost extent of glacial advance into the South Central US.

Figure 6.14: This Sioux quartzite erratic is located in the center of Granite Street in McLouth, Kansas. When the town was originally laid out in the 1880s, workers were unable to move the boulder, so they simply paved the road around it.

By 18,000 years ago, the ice sheet was in retreat due to a slight warming of the climate. Though the ice sheet alternately moved forward and melted backward, overall it was on the retreat. Even during glacial advance, the glacier was always melting at its fringes. During times of glacial retreat, the ice sheet was not actually flowing backwards—the glacier continued to flow forward, but it was melting faster than it was advancing.

The Loess Hills

The Loess Hills of extreme western Iowa, extending into northwesternmost Missouri, are named after a glacial deposit formed of windblown rock flour: loess. This type of glacial feature is found in significant quantities in only a few places on Earth. These hills form narrow, 320-kilometer (200-mile) long, north-south bands immediately east of the Missouri and Mississippi River floodplains, and thinner deposits across Kansas and central Missouri (Figure 6.15). They were formed during several glacial/interglacial cycles when glaciers ground down the bedrock. Later, as the ice retreated, meltwater deposited the fine sediments in expansive mudflats. When the mudflats dried, strong westerly winds blew the sand into great dunes, and the finest material (silt and clay) was carried even farther in massive dust clouds. The dunes were eventually stabilized by vegetation and matured into hills, but their loose material is still easily eroded and carved. Slumping, mudslides, and undercutting caused by wind and water have produced steep slopes and a landscape of narrow ridges (Figure 6.16).

Figure 6.15: Thickness of loess deposits in the South Central US.

Figure 6.16: Landscape of the Loess Hills along the Missouri River in Atchison and Holt Counties, Missouri.

Much of the soil throughout the eastern South Central is composed, in part, of sediment blown from huge mudflats on the banks of the ancient Missouri River, which was a major channel for floods of glacial meltwater. In and around the South Central US, loess deposits occur along the bluffs of the Missouri and Mississippi rivers, and may form hills several hundred meters high. Often, exposed loess will form steep faces of fine silt. The loess can become the base of rich soils and is part of the basis for the “corn belt,” an intensively agricultural area spanning much of the Midwest and extending into the Dakotas, Nebraska, Kansas, and Missouri.

The Coastline and Glaciers

It may seem surprising that the very southernmost part of the South Central US, along the coastline, would be among the areas of the South Central most influenced by glaciers. The reason is not related to the action of the flowing glaciers themselves, but rather the amount of water stored in glaciers globally: during glacial advances, so much water (ultimately evaporated from the surface of the ocean) is trapped in glaciers that sea level can drop by over 100 meters (330 feet). Thus, as recently as 20,000 years ago, what are now bays and river mouths along the coast of Texas and Louisiana were dry land many kilometers from the shore, as was much of the continental shelf that rims the coastline (Figure 6.17).

Figure 6.17: The coastline of the South Central US was considerably broader during the Pleistocene, when sea level drop exposed the continental shelf.

During each interglacial period, as sea level rose, the nature of the coastline was influenced by the erosion that had taken place in the previous glacial period, particularly in river valleys. For example, the Mississippi River cut more deeply into the land and transported more sediment during intervals of lower sea level, and, conversely, slightly higher floodplains from previous interglacial periods can be observed along the edge of the existing Holocene floodplain. The Missouri and Mississippi were conduits for some of the melting of the continental ice sheets; this meltwater contained glacially eroded sediments that contributed greatly to the Mississippi’s broad floodplains and delta.

As sea level rose to near current levels, it took time for sediments to accumulate just offshore and for the system of barrier islands, lagoons, bays, and estuaries that we know today to develop. The evolution of the coastline and associated changes over the past ten thousand years influenced the history of ecosystems and human settlement of the area.